Extending the range of Low Energy Electron Diffraction (LEED) surface structure determination: Co-adsorbed molecules, incommensurate overlayers and alloy surface order studied by new video and electron counting LEED techniques
Abstract
LEED multiple scattering theory is briefly summarized, and aspects of electron scattering with particular significance to experimental measurements such as electron beam coherence, instrument response and phonon scattering are analyzed. Diffuse LEED experiments are discussed. New techniques that enhance the power of LEED are described, including a real-time video image digitizer applied to LEED intensity measurements, along with computer programs to generate I-V curves. The first electron counting LEED detector using a wedge and strip position sensitive anode and digital electronics is described. This instrument uses picoampere incident beam currents, and its sensitivity is limited only by statistics and counting times. Structural results on new classes of surface systems are presented. The structure of the c(4 x 2) phase of carbon monoxide adsorbed on Pt(111) has been determined, showing that carbon monoxide molecules adsorb in both top and bridge sites, 1.85 + or - 0.10 A and 1.55 + or - 0.10 A above the metal surface, respectively. The structure of an incommensurate graphite overlayer on Pt(111) is analyzed. The graphite layer is 3.70 + or - 0.05 A above the metal surface, with intercalated carbon atoms located 1.25 + or - 0.10 A above hollow sites supporting it. The (2 sq. rt. of 3 x 4)-rectangular phase of benzene and carbon monoxide coadsorbed on Pt(111) is analyzed. Benzene molecules adsorb in bridge sites parallel to and 2.10 + or - 0.10 A above the surface. The carbon ring is expanded, with an average C-C bond length of 1.72 + or - 0.15 A. The carbon monoxide molecules also adsorb in bridge sites. The structure of the (sq. rt. of 3 x sq. rt. of 3) reconstruction on the (111) face of the alpha-CuAl alloy has been determined.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- November 1986
- Bibcode:
- 1986PhDT.........1O
- Keywords:
-
- Electron Scattering;
- Adsorption;
- Chemisorption;
- Electron Diffraction;
- Ion Beams;
- Surface Properties;
- Benzene;
- Carbon Monoxide;
- Chemical Composition;
- Copper Alloys;
- Electron Beams;
- Electron Guns;
- Graphite;
- Platinum;
- Rhodium;
- Topology;
- Solid-State Physics